We report the first quantitative analysis of spatially resolved local Pt distribution in graphite and carbon nanofibers supported PtRu alloy nanoparticles using slow-beat measurements by in situ Pt-195 electrochemical NMR. While the two samples had the same nominal 1:1 atomic ratio and similar average particle size, it was found that the former had a rather homogeneous Pt distribution but not the latter, which had Pt segregation at the core and depletion at the surface of the nanoparticles.